The message means what it says: Gmail will eventually accept the email, but it will be delayed.

I forward emails to my gmail account so that I have important messages in two places, and I recently noticed that this was causing these messages in my logs. As I am only forwarding to myself, I'm certainly not complaining to Google about spam, but of course I may be forwarding spam that I get myself. Was that why I got those messages?

At the page referenced above, Google makes several recommendations:

To ensure that Gmail can identify you:

Use a consistent IP address to send bulk mail.

Keep valid reverse DNS records for the IP address(es) from which you send mail, pointing to your domain.

Use the same address in the 'From:' header on every bulk mail you send.

We also recommend the following:

Sign messages with DKIM. We do not authenticate messages signed with keys using fewer than 1024 bits.

I felt I was meeting all of those recommendations except DMARC, so I added that. Still, the messages kept cluttering my INBOX. What was going on?

I was astounded when I found what may be the answer: a while back I had moved my mailserver to a new host and IP address. Amazingly, I forgot to have a PTR (reverse DNS) record created!

Of course I corrected that immediately. I don't know yet if that was the last piece Google needed, but it may well have been. It would certainly have helped if they were more specific in their responses!